A laser induced fluorescence technique for determining the pH of water droplets and probing uptake dynamics

A new experimental approach is demonstrated for probing the uptake dynamics of aerosols. By seeding liquid aerosol droplets with a pH sensitive dye, the laser induced fluorescence (LIF) spectrum can be used to unambiguously and directly determine the droplet pH. Droplets larger than 15 mm in radius are studied in this work. Measurements show that for these size droplets, the fluorescence spectrum is independent of droplet size and conforms to that obtained in bulk phase measurements. Fluorescence lifetime measurements support the conclusion that the measurements are inherently bulk phase in character. By using cavity enhanced Raman scattering, the size of the droplets being sampled is accurately determined. The velocity of the droplets is determined from elastic light scattering measurements. In a droplet train apparatus in which the gas phase composition can be accurately controlled, the LIF technique is used to investigate the uptake of ammonia by acidic water droplets with varying ammonia partial pressure and exposure time.